<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="+" onclick="f(1)">
<input type="button" value="-" onclick="f(2)">
<input type="button" value="*" onclick="f(3)">
<input type="button" value="/" onclick="f(4)">
<div>
    结果:<span>0</span>
</div>
<script>
    function f(x) {
        //得到页面中的元素对象
        let i1 = document.querySelector("#i1");
        let i2 = document.querySelector("#i2");
        let s = document.querySelector("span");

        //判断输入的值是不是NaN
        if (isNaN(i1.value) || isNaN(i1.value)) {
            s.innerText = "输入错误!";
            return;
        }

        switch (x) {
            case 1:
                s.innerText = i1.value*1 + i2.value*1;
                break;
            case 2:
                s.innerText = i1.value - i2.value;
                break;
            case 3:
                s.innerText = i1.value * i2.value;
                break;
            case 4:
                s.innerText = i1.value / i2.value;
        }
    }
</script>
</body>
</html>